I need to detect 0Vdc or >60Vdc (system on).
Assuming that you need isolation.
- Get the optocoupler's datasheet. Find the current needed to light up the led.
- Get a resistor that will generate that current under 60v. You can size that resistor a little bit higher if you want to make sure that there is sufficient threshold.
- If you want to protect the opto-coupler for reverse polarity, put a diode on it, or another opto-coupler but reverse the polarity - in that case, you can detect ac as well as dc.
- Done.